1. Field of the Invention
The present invention relates to an improved method for the separation of different types of plastics, and more particularly to an improved process for the separation of acrylonitrile butadiene styrene (ABS) and high impact polystyrene (HIPS) plastics from each other.
2. Description of the Prior Art
Recycling of different types of plastics has increased in recent years. Processes for separating different plastic types have become increasingly important. For example, "Recovering Plastics for Recycling by Mineral Processing Techniques," by R. Buchan and B. Yarar, JOM, February, 1995, pps. 52-55, discloses a process for separating plastics by use of a mineral processing technology.
U.S. Pat. No. 5,399,433, issued Mar. 21, 1995 to Kobler, discloses a method for separating a polyethylene terephthalate (PET)/polyvinyl chloride (PVC) chip admixture. The PET/PVC chip admixture is contacted with a surface conditioning agent to produce relatively hydrophobic polyvinyl chloride chips which can be floated in an aerated aqueous medium. The polyethylene terephthalate chips are recovered from the bottom of the aqueous medium. The flotation is conducted in pure water without frothing or densifying agents.
Separation of solids using differences in their densities is a simple, economical and effective technique. A liquid whose density is between the densities of two solids can be used as the working medium. The solid with the lesser density floats and the solid with the higher density sinks resulting in the separation of the mixture. However, materials that have similar densities, such as acrylonitrile butadiene styrene, ABS, and high impact polystyrene, HIPS, can not be separated in high purities by this simple and inexpensive technique.
Different grades of both of the ABS and HIPS plastics have specific gravities in the range of 1,055 to 1.125. As a result such mixtures can not be effectively separated by density gradient procedures. Attempted separation of ABS and HIPS plastics by the inventors in their laboratory using conventional density gradient procedures has produced ABS product that is about 95% pure. However, 5% HIPS impurity in the ABS resulted in severe degradation of its properties, specifically its tensile and impact strengths because the two plastics are not compatible.
The recycling of white goods such as refrigerators in the United States and Europe involves the shredding of the refrigerators after the removal of the bulk refrigerants. The metallic content of the shredded material is then separated and recycled, and the non-metallic residue is considered waste, and is presently land filled. The two dominant plastics in refrigerators are acrylonitrile butadiene styrene (ABS) and high impact polystyrene HIPS. Other plastics may also be present, such as polypropylene, polyethylene, nylons and polyvinyl chloride. Many of these plastics can be separated from each other and from HIPS and ABS utilizing differences in density. Because ABS and HIPS have similar densities, ABS and HIPS are not separable by this method. ABS and HIPS are not compatible and a small amount of contamination of one in the other severely restricts its commercial value. One method that we successfully tested for separating HIPS from ABS takes advantage of the selective stickiness of the materials generated by solvents. While this method may be effective in separating the two plastics, the use of quantities of organic solvents has environmental drawbacks. Another technique that may be used for separating HIPS and ABS involves contacting the mixture with a surface that is heated to a temperature high enough for HIPS to stick to the hot surface but not hot enough for ABS to become sticky. Continuous cleaning of the hot surface is necessary to prevent sticking of the ABS to spots on the hot surface that have residual sticky HIPS.
It is an object of the present invention to provide an improved method process for the separation of acrylonitrile butadiene styrene (ABS) and high impact polystyrene (HIPS) plastics from each other.
It is another object of the present invention to provide such an improved method for separating ABS and HIPS plastics without requiring the use of organic solvents, and at ambient conditions to minimize energy consumption.
It is another object of the present invention to provide such an improved method that provides reliable and effective operation.
It is another object of the present invention to provide such an improved method that overcomes many of the disadvantages of prior art arrangements.